J. Walter Strapp

Assessing Cloud-Phase Conditions

Abstract In situ microphysics measurements made during the First and Third Canadian Freezing Drizzle Experiments (CFDE I and III, respectively) have been used to assess the relative responses to ice and liquid hydrometeors for several common instruments. These included the Rosemount icing detector, 2D-C monoscale and 2D-C grayscale probes, forward-scattering spectrometer probes (FSSP) on three measurement ranges, Nevzorov liquid water content (LWC) and total water content probes, and King LWC probes. The Nevzorov LWC and King LWC probes responded to between 5% and 30% of the ice water content, with an average response of approximately 20%. The average FSSP measurements of droplet spectra were dominated by ice particles for sizes greater than 35 μm, independent of the measurement range used, when the ice-crystal concentrations exceeded approximately 1 L−1. In contrast, the FSSP measurements of the droplet spectra less than 30 μm appeared free of ice-crystal contamination, independent of the ice-crystal con...

Characterizations of Aircraft Icing Environments that Include Supercooled Large Drops

Abstract Measurements of aircraft icing environments that include supercooled large drops (SLD) greater than 50 μm in diameter have been made during 38 research flights. These flights were conducted during the First and Third Canadian Freezing Drizzle Experiments. A primary objective of each project was the collection of in situ microphysics data in order to characterize aircraft icing environments associated with SLD. In total there were 2793 30-s averages obtained in clouds with temperatures less than or equal to 0°C, maximum droplet sizes greater than or equal to 50 μm, and ice crystal concentrations less than 1 L−1. The data include measurements from 12 distinct environments in which SLD were formed through melting of ice crystals followed by supercooling in a lower cold layer and from 27 distinct environments in which SLD were formed through a condensation and collision–coalescence process. The majority of the data were collected at temperatures between 0° and −14°C, in stratiform winter clouds assoc...

Extratropical Transition of Hurricane Michael: An Aircraft Investigation

In order to better understand the behavior and impacts of tropical cyclones undergoing extratropical transition (ET), the Meteorological Service of Canada (MSC) conducted a test flight into Hurricane Michael. Between 16 and 19 October 2000 the transition of Hurricane Michael from a hurricane to an intense extratropical storm was investigated using a Canadian research aircraft instrumented for storm research. This paper presents the various data collected from the flight with a detailed description of the storm structure at the time when Michael was in the midst of ET. Hurricane Michael was moving rapidly to the northeast, approximately 300 km southeast of Nova Scotia, Canada, during the time of the aircraft mission. A period of rapid intensification had also occurred during this time as the system moved north of the warm Gulf Stream waters and merged with a baroclinic low pressure system moving offshore of Nova Scotia. Consequently, the hurricane was sampled near the period of its lowest surface pressure ...

In situ measurements of effective diameter and effective droplet number concentration

The effective diameter of cloud droplets is usually derived from measurements of droplet size distribution measured by Particle Measuring Systems (PMS) probes. The disadvantage of this method is that PMS probes have a truncated size range. During the RACE project, an alternative method to measure the effective diameter used a cloud extinction meter and King and Nevzorov hot wire liquid water content and total water content (LWC/TWC) probes installed on the National Research Council (NRC) Twin Otter. The effective diameter was derived from direct in situ measurements of the extinction coefficient (e) and liquid water content (W) as Deff=k1W/e. This method of calculation of Deff is free of problems related to deriving Deff from the truncated particle size distribution. Since measurements of e and W cover the whole size range of cloud particles, this method gives an accurate value of Deff. This method can also be successfully applied for mixed and ice phase clouds, since the Nevzorov TWC probe provides measurements of total (ice plus liquid) water content. Effective number concentration was calculated as Neff = k2ϵ3/W2. Comparisons of Deff and Neff, calculated by this method, and directly from PMS Forward Scattering Spectrometer Probe (FSSP) spectra, are favorable in the subset of conditions when the FSSP is considered to measure the spectra fully and accurately.

Assessing the Collection Efficiency of Natural Cloud Particles Impacting the Nevzorov Total Water Content Probe

The Nevzorov Total Water Content Probe has been used extensively for characterizing the cloud water content (ice and liquid) in clouds where airframe icing and engine related problems might occur. Because of recent work done in the ice simulating wind tunnels, where a significant fraction of ice particles were observed to be ejected from the sensor instead of captured and evaporated, it is suspected that the Nevzorov Total Water probe and other similar hot-wire sensors are providing underestimates of ice water content and possibly even liquid water content in the presence of large drops. In-flight tests were performed in December 2004 where natural ice particles (dendritic ice crystals and aggregates) and water drops were photographed impacting a specially mounted Nevzorov probe installed on the NRC Convair-580. A high speed camera captured small fragments of ice particles and water drops being swept out of the collecting cone of the total water sensor after impact. It appears that the Nevzorov probe is detecting a decreasing fraction of the mass of individual particles (solid and liquid) with increasing particle size, although this effect could not be quantified with the available data. It should be emphasized that even when ice crystal shattering was occurring with subsequent loss of mass, the Nevzorov probe was still detecting a fraction of the ice. One of the interesting observations was the apparent difference in the visual impression of natural particle impacts during the flight tests versus simulated ice impacts in the wind tunnel tests. Particles did not “bounce” out of the sensor as they appeared to in the tunnel experiments, but appeared to shatter into small fragments, some of which would be ejected from the sensor in a complicated manner. This difference is postulated to be due to the higher density of simulated ice particles in the tunnel. The current tests documented impacts with natural cloud ice particles that would be classified as “fragile” and usually with a low density. The results suggest that both wind tunnel and insitu tests are needed to fully describe probe performance in ice particle environments.

Regular airborne surveys of Arctic sea ice and atmosphere

The Arctic is undergoing rapid environmental change, manifested most dramatically by reductions in sea ice extent and thickness. The changes are attributed to anthropogenic effects related to greenhouse warming, with secondary contributions from changing ocean and wind currents as well as from pollutants, especially “absorbing” black carbon. The warmer Arctic air temperatures and new patterns of wind and ocean circulation have also contributed to a younger ice cover [Maslanik et al., 2011]. Specific factors that determine the temporal distribution of sea ice are poorly understood because few observations of key variables have been made in the central Arctic. For example, the planetary boundary layer (PBL), the lowest part of the atmosphere governed by interaction with Earths surface, plays a critical role involving the exchange of momentum, heat, water vapor, trace gases, and aerosol particles. Satellites can provide limited observations of sea ice properties, but so far, accurate measurements of ice thickness or boundary layer properties have not been easily obtained. Although satellite retrievals of geophysical variables might be an essential source of information, their reliability remains questionable owing to inadequate spatial and/or temporal resolution and to a need for further validation.

HAIC/HIWC Field Campaign - Specific Findings on PSD Microphysics in High IWC Regions from In Situ Measurements: Median Mass Diameters, Particle Size Distribution Characteristics and Ice Crystal Shapes

Despite past research programs focusing on tropical convection, the explicit studies of high ice water content (IWC) regions in Mesoscale Convective Systems (MCS) are rare, although high IWC conditions are potentially encountered by commercial aircraft during multiple in-service engine powerloss and airdata probe events. To gather quantitative data in high IWC regions, a multi-year international HAIC/HIWC (High Altitude Ice Crystals / High Ice Water Content) field project has been designed including a first field campaign conducted out of Darwin (Australia) in 2014. The airborne instrumentation included a new reference bulk water content measurement probe and optical array probes (OAP) recording 2D images of encountered ice crystals. The study herein focuses on ice crystal size properties in high IWC regions, analyzing in detail the 2D image data from the particle measuring probes. Various geometrical parameters were extracted from the images in order to calculate particle size distributions (PSDs) and finally deduce median mass diameters with additional information on the ice density. The preliminary analysis of all HAIC/HIWC flights performed during this first flight campaign out of Darwin, demonstrates that various flights include high IWC regions mostly produced by high concentrations of small crystals while other flights with similar peak IWCs indicates that high IWC regions could be nevertheless composed primarily of larger particles. This interesting result indicates that high IWC can be produced and maintained in various environments, preferentially high concentrations of small crystals, however sometimes by smaller concentrations of larger sized crystal populations.

Aircraft Escape Strategy from Supercooled Cloud Layers

Vertical profiles of liquid water in supercooled frontal stratiform clouds have been studied in order to estimate the potential rate of ice accretion at different levels within the cloud and to develop recommendations for escape strategies to avoid severe in-flight icing. The vertical soundings of the supercooled liquid clouds were obtained using the National Research Council of Canada Convair-580 equipped by Environment Canada for cloud microphysical measurements. The data were collected during five flight campaigns (CFDE 1, CFDE 3, AIRS 1, AIRS 1.5 and AIRS 2). In total 584 vertical LWC profiles were analyzed. A statistical summary has been prepared from the profiles of the potential thickness of accreted ice, liquid water content, temperature, and cloud depths. The maximum potential accreted thickness of ice does not exceed 2cm for a transit with a 3 degree glide slope thoughout the cloud depth. Based on the statistics, in order to avoid severe icing once significant icing is encountered, it is suggested that a climb or descent should be initiated. The aircraft should not stay at the same altitude within the cloud layer. I. Introduction T is difficult for pilots to decide to climb, descend, or stay at the same level when encountering in-flight icing, especially in icing events associated with drizzle. During these periods the airplane may have to traverse one or several supercooled cloud layers and as a result experience in-flight icing. To estimate the potential consequences of in-flight icing, liquid water content (LWC) profiles in supercooled cloud layers associated with frontal systems were analyzed. The vertical distribution of liquid water and temperature inside supercooled clouds is important for estimation of icing severity during any traverse of such layers. The statistical characteristics of supercooled stratiform frontal clouds collected during five aircraft field campaigns are presented here. Effort was focused on an analysis of the statistics of cloud depths, vertical distribution of liquid water and temperature inside clouds, and the potential thickness of accreted ice. The results provide an overview of conditions that pilots may encounter in mid and high latitude winter clouds. Based on these results suggestions have been made to minimize in-flight icing and avoid severe icing during climb and descent.

Measuring Cloud Parameters for In-Flight Icing Certification Tests

It is often necessary to make cloud measurements for in-flight certification of new aircraft or equipment that will operate in icing conditions. However, aircraft manufacturers and regulatory agencies do not have much guidance in terms of how to conduct such measurements. This paper explores the range of equipment currently available and makes recommendations as to the practices that should be followed. The rationale for selecting equipment, the proper mounting of probes, the calibration of probes, and the analysis of data are examined. Because technology is rapidly evolving, no specific instruments are recommended, although the limitations of some older types of equipment will be documented. Certification tests within Appendix C, and within a possible newly specified supercooled large drop (SLD) environment, will be considered. Such guidance will hopefully provide manufacturers and regulators a means of evaluating their current and future practices.